Known in the art is an acoustic wave element having a piezoelectric substrate and excitation IDT electrode (InterDigital Transducer) electrode provided on the main surface of the piezoelectric substrate. Such an acoustic wave element is for example utilized for a transmission filter, reception filter, and so on in a duplexer.
In an acoustic wave element, an electrical distorted signal (distorted wave) which is generated due to nonlinearity of the element sometimes may cause the electrical characteristics to fall. For example, in a duplexer using an acoustic wave element, an out-of-band interfering wave out of the transmission band and reception band and a transmission wave may be mixed, so a distorted wave contained in the reception band may be generated. This distorted wave is called “inter-modulation distortion (IMD)” and may be one of the causes of a drop in the communication quality (SN ratio) of wireless communication apparatuses. Other than this, there also may exist the possibility of the problem that harmonic distortion having a frequency of a whole multiple of the transmission wave is generated and this harmonic distortion hinders the communication of other wireless communication apparatuses.
As a method for reducing the drop in the SN ratio due to such a distorted wave, there is known the method for dividing the serial resonators or parallel resonators of a ladder type filter configuring the duplexer without changing the electrostatic capacity. This reduces the distorted wave by dividing the serial resonators or parallel resonators and thereby dispersing the voltage applied to the resonators.